LONDON — In a major setback for British prosecutors, a jury on Wednesday acquitted five former brokers of charges that they helped a onetime trader at UBS and Citigroup manipulate an important benchmark interest rate known as Libor.

The London jury is continuing to deliberate over one remaining count of conspiracy to defraud against a sixth broker, Darrell P. Read, who worked at the British financial firm ICAP, after it was unable to reach a verdict. The jury went home for the day on Wednesday and will return Thursday morning.

The jury found Mr. Read not guilty on a separate conspiracy count on Wednesday.

The acquittals, coming just a day after the jury began deliberating, are a blow to British authorities, who have been criticized for not being as aggressive as the United States Justice Department in prosecuting financial crimes. The trial of the six began in October and is the second in Britain to focus on the Libor scandal.

The two trials have followed a half-decade investigation that has led to billions of dollars in fines, cost one chief executive his job and damaged the reputations of some of the world’s biggest banks, including Barclays, the Royal Bank of Scotland, UBS and Deutsche Bank.

The former brokers, who worked at the British financial firms ICAP, RP Martin and the Tullett Prebon Group, were accused of conspiring to manipulate the London interbank offered rate, or Libor.

Prosecutors had accused the men of helping Tom Hayes, a former trader at UBS and Citigroup, and others profit by rigging Libor, which helps determine the borrowing costs for trillions of dollars in loans.

At a separate trial, Mr. Hayes was convicted in August of conspiracy to defraud. In December, a British appeals court reduced his sentence to 11 years in prison from his original sentence of 14 years.

Mr. Hayes was the first person to go on trial in Britain over criminal charges related to Libor manipulation, and his case was seen as a bellwether for efforts by British authorities to pursue financial crime.

“The key issue in this trial was whether these defendants were party to a dishonest agreement with Tom Hayes. By their verdicts, the jury have said that they could not be sure that this was the case,” David Green, the director of the Serious Fraud Office, which brought the criminal case, said in a statement.

“Nobody could sensibly suggest that these charges should not have been brought and considered by a jury,” he added.

A third trial of other people accused of manipulating Libor is expected to begin as early as next month.

Lawyers for the six former brokers on trial in London argued that the men were being made scapegoats for a flawed system and that they had lied to Mr. Hayes about their ability to influence the rate.

The fraud office accused the brokers — several of whom had colorful nicknames — of helping Mr. Hayes and others try to manipulate submissions of Libor as it related to the Japanese yen by so-called panel banks, financial institutions that are surveyed each day and whose information is used to calculate Libor.

To set Libor and other rates, banks submit the rates at which they would be prepared to lend money to one another, on an unsecured basis, in various currencies and at varying maturities.

For example, prosecutors said, Mr. Goodman would make predictions each morning on where Libor would be fixed later that day, earning him the nickname Lord Libor.

Prosecutors argued at trial that Mr. Goodman had been aware of Mr. Hayes’s “wishes” on Libor and skewed his daily predictions about where Libor related to the yen would be fixed to manipulate the interest rate.

Mr. Read, called Big Nose, was Mr. Hayes’s main contact at ICAP and served as a link to Mr. Goodman and Mr. Wilkinson, prosecutors said. Mr. Wilkinson, called Sarge, was head of the desk where Mr. Read worked.

The trial revolved around a series of emails, instant messages and other documents related to the complicated process of setting Libor, often in the colorful language favored by traders.

A version of this article appears in print on , on Page B1 of the New York edition with the headline: Jury in London Clears 5 Ex-Brokers in Libor Trial, a Blow to Prosecutors. Order Reprints | Today’s Paper | Subscribe